A key to understanding the function of any tissue is the biochemical characterization of the proteins that are specific to that tissue. The mammalian brain is composed of two predominant cell types, neurons and glia, which are organized into a great variety of structures. Whether the brain is viewed as one or several tissues, its organization involves many cells that are believed generally to perform the same physiological actions but at different places and in response to different signals. Accordingly, one can expect to find a large number of brain-specific proteins in a mature brain, including those involved in both specialized and general brain processes.
Exemplary of specifiable brain-specific proteins are: neuropeptide precursors, enzymes responsible for neurotransmitter synthesis and/or processing, proteins that participate in the release, degradation or reuptake of neurotransmitters, signal receptor systems and ion channels. Proteins included in the basic cellular structures include those peculiar to neurons (axons, dendrites and synapses) and those involved in establishing specific cell-cell interactions. Also, there will be proteins involved in mental processes such as memory which are not at all yet understood at the cellular, let alone molecular level. Clearly, the brain is a difficult tissue to study at the molecular level due to its great complexity.
Conventional protein chemistry is unsuitable for defining rare, unique proteins or structures present in very small regions of the brains. Furthermore, approaches at the protein chemical level are dependent on having a suitable assay for each protein, which is usally not generalizable to other molecules, and which approach will clearly only be applicable to molecules that fit an existing conceptual framework, such as the hypophysiotrophic factors. Guillemin, Science, 202, 390-402 (1978).
It is presumed, however, that all of the proteins of the brain are synthesized by translation from specific messenger RNA (mRNA) molecules, and thus each brain-specific protein must have a corresponding mRNA. Thus, one approach to the study of the brain is through the transcription pattern of messenger RNA (mRNA) molecules, or by asking the question of what messenger RNA species the brain produces relative to its complete genomic potential. Estimates for the complexity of mammalian brain-specific mRNAs are very high; tens to hundreds of thousands of discrete mRNA molecules are implicated in brain function [Bantle et al., Cell, 8, 39-150 (1976) and Hastie et al., Cell, 9, 761-774 (1976)], consistent with the variety of brain-specific proteins listed above.